1. Field of the Invention
The present invention relates to an optical printer head and a driving method thereof, and more particularly to an optical printer head and a driving method thereof for use in exposure of a photosensitive member to light in an electrophotographic printer.
2. Description of the Prior Art
Conventionally, an optical printer using a line light source and a laser printer are known as electrophotographic printers. The laser printer modulates laser light in accordance with output data to produce modulated laser light which scans on a photosensitive drum using a plurality of lens systems and a polygon mirror to form an image with the light on the photosensitive drum before development of the image, thereby producing printed output.
The laser printer allows a faster speed, a higher quality image, and a lower noise level than a dot impact printer and an ink-jet printer. The laser printer is not only used for business as a printer capable of printing on plain paper, but also becoming prevalent for home use in recent years.
The optical printer with a line light source employs a line light source including light-emitting elements placed in a line, and has an advantage of no need of a scanning optical system since the aligned light-emitting elements irradiate associated spots on a photosensitive member with light, respectively. Thus, the optical printer with the line light source can realize higher reliability and a smaller size of a printer apparatus.
FIG. 1 shows a general configuration of a conventional optical printer using a line light source. In FIG. 1, the conventional optical printer generally comprises data input section 51 for receiving printing data, photosensitive drum 27, optical printer head 21 for exposing photosensitive drum 27 to light in accordance with image data, converging rod lens array 24 for forming an image with the light from optical printer head 21 on photosensitive drum 27, charger 42, cleaner 25, developer 43, transferrer 44, and charge eliminator 45. Charger 42, cleaning section 25, developer 43, transferrer 44, and charge eliminator 45 are placed to surround photosensitive drum 27.
The operation of the conventional optical printer using the line light source is hereinafter described with reference to FIG. 1.
Image data output from data input section 51 is input to a driving circuit of optical printer head 21. The output from the driving circuit causes the line light source in optical printer head 21 to emit light. The light emitted from the activated optical printer head 21 is converged by converging rod lens array 24 and irradiated to photosensitive drum 27. The surface of photosensitive drum 27 has been uniformly charged by charger 42 such that electric charge is removed in the portions irradiated with the light from optical printer head 21 to write an electrostatic latent image on photosensitive drum 27.
Developer 43 sprays charged particles (toner) onto the surface of photosensitive drum 27 with the electrostatic latent image written thereon to develop the electrostatic latent image, thereby forming a toner image. The toner image reaches object 26 of printing such as a sheet of paper with the rotation of photosensitive drum 27, and is transferred onto object 26 of printing through the application of an electric field by transferrer 44. The transferred toner image is fixed on object 26 of printing by a fixer (not shown).
The residual electric charge on the surface of photosensitive drum 27 after it passes through transferrer 44 is removed by charge eliminator 45, and finally, cleaner 25 removes the toner remaining on the surface of photosensitive drum 27 after the transfer.
As a light source of such an optical printer, for example, Japanese Patent Laid-open Publication No. Sho 58-65682 (JP, 58065682, A) discloses the use of a light source including a number of LEDs (Light Emitting Diodes) placed in a line.
A ceramic substrate formed of alumina is primarily used as a substrate of a printer head using LEDs. The printer head is formed by placing a plurality of LED chips in a line on the ceramic substrate, performing die bonding of IC (Integrated Circuit) chips serving as driving circuits on both sides of the LED chips with conductive paste, and then making electrical connection through wire bonding. The ceramic substrate of the printer head is supplied with electric signals and power from the main body of the printer through an FPC (Flexible Printing Cable).
For the LED chips in this case, an array of LEDS for 64 dots or 128 dots and with approximately 60 xcexcm pitches is currently used in consideration of limitations on the size of an n-type GaAsP substrate for forming part of the LED chips, yields in the manufacturing process, and the like. A plurality of such LED chips need be arranged to form a line light source of a printer head, and in such a case, highly accurate cutting technique and mounting technique on the order of micrometers are required to increase the accuracy of the arrangement.
In addition, since the n-type GaAsP substrate used is small, expensive, and even causes many defects, an attempt to increase the number of dots for light emission in a monolithic LED chip leads to reduced yields and significantly increased manufacturing cost. A method of avoiding these problems is to mass-produce LED chips for a reduced number of dots which are aligned over the length covering the printing width corresponding to an object of printing. This method, however, has mounting limitations from the issues in the arrangement of the chips and the electrical connection when a higher density is intended. As a result, the optical printer with the LEDs has limitations in providing a lower cost and higher density.
To address such problems, the use of a light-emitting element other than the LED is considered. For example, Japanese Patent Laid-open Publication No. Hei 8-108568 (JP, 08108568, A) discloses an optical printer head using an organic EL (electroluminescence) thin film light-emitting element. Since the optical printer head using the organic EL light-emitting element enables the formation of a number of light-emitting elements together on a substrate of large area as well as mass production, a lower cost can be expected. In addition, a higher density is possible due to micromachining of electrode sections in the manufacturing process.
Light-emitting elements are arranged two-dimensionally in an optical printer head to allow exposure to light in a short time even if they emit light at low luminance. For example, Japanese Patent Laid-Open Publication No. Hei 9-254437 (JP, 09254437, A) describes a printer having a printing head in which light-emitting elements are arranged two-dimensionally and a pixel array using a group of optical fibers as its front panel is used.
However, for the optical printer head using a thin film light-emitting element such as an organic EL element, the performance of the current organic EL element has a limitation of emitting light up to a luminance of several hundreds of cd/m2 assuming that its useful life is several tens of thousands hours. In other words, when the current organic EL element is used, it is difficult to meet both needs for the amount of light required for exposure as a printer head and for the practical life. The practical life refers to the maximum number of sheets of paper required for use as a printer.
It is contemplated that a replaceable optical printer head is employed to allow light emission at a higher luminance at the expense of the useful life. However, it is difficult for a user to align a new optical printer head with a photosensitive drum and optical systems on the order of micrometers in replacing an old optical printer head.
Problems common to electrophotographic printers include the need of correction for sensitivity characteristics of a photosensitive member, the need of correction for misalignment of an object of printing, the need of correction for insufficient development in an area exposed to a small amount of light when multi-level gradation printing is performed, and the like.
Of these corrections, as to the correction for sensitivity characteristics of a photosensitive member, the characteristics of potentials on the surface of the photosensitive member with respect to an amount of light exposure is not necessarily linear, and as a result, the operation of the printer need be performed in accordance with the sensitivity characteristics of the photosensitive member. The correction of misalignment of an object of printing must be performed without exception since the misalignment causes reduced printing quality. The problem of insufficient development in an area exposed to a small amount of light generally occurs in a conventional photosensitive member, and requires certain countermeasures similarly to the other two problems.
When light-emitting elements are arranged two-dimensionally, an increased number of light-emitting elements causes increases in driving circuits, wiring and the like provided outside the optical printer head, thereby making it difficult to achieve a higher density and a smaller size. The two-dimensionally arranged light-emitting elements are referred to as a pixel array. A method of driving a printer using the pixel array requires rewriting of data in all the pixels of the pixel array during a main scanning period. This results in the need of serial input of printing data to all pixel columns during the main scanning period. Thus, the realization of a printer capable of fast printing involves a high driving frequency required for its driving circuits.
It is an object of the present invention to provide an optical printer head which can be driven with a driver IC with a low driving frequency and which can easily realize a higher density, a smaller size, and faster printing.
It is another object of the present invention to provide a method of driving an optical printer head which can be driven with a driver IC with a low driving frequency and which can easily realize a higher density, a smaller size, and faster printing.
The first object of the present invention is achieved by an optical printer head comprising: a pixel array including pixels arranged two-dimensionally in a row direction and a column direction, each of the pixels including a light-emitting element; a memory array including memory cells arranged two-dimensionally in a row direction and a column direction for holding printing data input thereto; a horizontal scanning circuit for supplying a data signal to each memory cell column in the memory array; a first vertical scanning circuit for sequentially selecting memory cell rows to write binary data to each memory cell in the memory array; a selecting circuit for arbitrarily selecting the memory cell rows to read binary data from each memory cell in the memory array; a second vertical scanning circuit for sequentially selecting pixel rows in the pixel array; and a buffer located on a transfer path between the memory array and the pixel array.
The optical printer head of the present invention may further comprise a switch array located on a transfer path between the horizontal scanning circuit and the memory array. Alternatively, the optical printer head may comprise a first switch array located on a transfer path between the horizontal scanning circuit and the memory array and a second switch array located on a transfer path between the memory array and the pixel array.
In the present invention, the respective circuits are preferably formed on a single insulator substrate, and each of them is preferably formed of a polycrystalline silicon thin film transistor.
In the present invention, an organic electroluminescence element may be used as the light-emitting element, for example.
According to the optical printer head of the present invention, a higher density, a smaller size, and faster printing are possible. Also, since a plurality of the light-emitting elements in the vertical scanning direction can expose the same spot on the photosensitive member to light a plurality of times, it is possible to achieve exposure to a desired amount of light even when the light-emitting elements emit a small amount of light. Therefore, according to the present invention, the optical printer head can be driven even with a driver IC driven at a low frequency in reading from the memory array.
In another aspect of the present invention, the horizontal scanning circuit may comprise a shift register, a data register, a latch, and a buffer. With such a configuration, only a start signal including a single pulse is transmitted within the shift register, which reduces the influence of a resistance load and a capacitive load impairing the transmission. Thus, the present invention can provide the optical printer head capable of faster input and transfer of printing data.
In another aspect of the present invention, the pixel array in the optical printer head may be divided into a plurality of groups of sub-pixels each comprising a plurality of sub-pixels in the same row direction and the same column direction, and the vertical scanning circuit for the pixel array may be configured to vary the number of sub-pixels emitting light among groups of sub-pixels. Such a configuration enables multi-level gradation printing with binary data as input.
The optical printer head of the present invention may comprise a first switch array and a second switch array at the input and output of the memory array, respectively. Such a configuration allows operations similar to those in the aforementioned configurations even when the memory cell includes one data line serving both as a writing data line and a reading data line.
In a yet another aspect of the present invention, the optical printer head comprises the data input buffer and the first switch array on a transfer path between the horizontal scanning circuit and the memory array, and the sense amplifier and the second switch array on a transfer path between the memory array and the pixel array. It is thus possible to perform operations similar to those in the aforementioned configurations with a static RAM (Random Access Memory) used for the memory cell.
The second object of the present invention is achieved by a method of selectively irradiating a surface of a rotatable photosensitive member with light in accordance with printing data using the aforementioned optical printer head of the present invention, comprising the steps of: in a state where pixel rows in the pixel array are in parallel to a rotational axis of the photosensitive member and the light-emitting element emits light in a direction opposed to the surface of the photosensitive member, establishing a state where a spot on the surface of the photosensitive member is irradiated with light or a state where the spot is not irradiated with light by means of one of light emission and no emission from an n-th pixel (where n is an integer equal to or greater than 1) in the pixel array; and establishing a state where the spot on the surface of the photosensitive member is irradiated with light or a state where the spot is not irradiated with light by means of one of light emission and no emission from an (n+1)-th pixel in the pixel array during a period in which the spot passes the (n+1)-th pixel.
In the driving method of the present invention, the photosensitive member is typically a photosensitive drum. A time period for the photosensitive drum to move over one pixel row in the pixel array is considered as a main scanning period, and it is preferable that (a) during the main scanning period, printing data input from the outside is input to the horizontal scanning circuit and the printing data is held in the latch circuit in the horizontal scanning circuit in response to a latch signal input from the outside, and (b) during the input of the printing data from the outside to the horizontal scanning circuit, printing data is read from the memory array and written to the pixel array to perform one of light emission and no emission sequentially in the light-emitting elements in the pixel array, and printing data for one row held in the horizontal scanning circuit is written to the memory cell.
In the driving method of the present invention, since data writing to the memory array from the horizontal scanning circuit and subsequent writing to the pixel array from the memory array are performed during the same horizontal scanning period, data for one row in the pixel array input in a horizontal scanning period can be written to the pixel array in the next horizontal scanning period.
As described above, according to the present invention, it is possible, in the optical printer head including a plurality of the light-emitting elements arranged two-dimensionally, to achieve exposure to a desired amount of light even when the light-emitting elements emit light at a low luminescence. In addition, according to the present invention, correction for sensitivity characteristics of the photosensitive member and correction for misalignment of an object of printing can be performed easily, and multi-level gradation printing can be performed with binary data. The memory array including a number of memory cells formed thereon for allowing writing, holding, and reading of data is provided on the same substrate, and printing data for all the pixels are held in and read from the memory array. Thus, according to the present invention, the optical printer head can be driven even with a driver IC driven at a low frequency, and it is possible to provide the optical printer head facilitating a higher density, a smaller size, and faster printing.
The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate examples of preferred embodiments of the present invention.